Treatment of paper



raa amein. 1

,. to E. I. du Pont de Nemours 8; Company, Wilv mington, Del., a corporation of Delaware N Drawing.

This invention relates to the manufacture of paper products such as cardboard or paperboard maybe impregnated'with solutions of 'methylol ureas at a slightly alkaline pH. and that the re- .sulting materials may be cured at" elevated tem-' perature and pressures. to produce products having' a hardened and dimensionally relatively stable surface.

It has been known fo many-years that fibrous materials, particularly paper, wood,-and thelike, may be impregnated with certain condensation products of urea and formaldehyde, and that.

urea-formaldehyde resins may be hardened with-v in the fibrous material after impregnation(Pol-.

; qlak, U. S. Patent, 1,459,153). Sincethe time-of this early disclosure severalpatents have de-, scribed the manufacture of various molding compositions obtained by treatment of cellulose or shredded paper with urea-formaldehyde condensation products. Generally, these compositions could be molded under heat and pressure. Also, processes have been known for impregnation I of paper with aqueous solutions of water-soluble urea-formaldehyde resins, generally in the presenceeof an' acid-reacting catalyst, and in certain instances, in the presence of an excess of the formaldehyde reactant.

mixed with the fibers prior to the formation. of

. traveling wire screens and thereafter passed over rolls which form the stock into a sheet.

- In contrast with the above processes the present 1 invention is directed to the manufacture of paper products which are not merely impregnated I throughout the mass of fibers with urea- 7 formaldehyde resin, but which have a continuous impregnated surface which protects the mass of fibers in the interior. Thus, the present invention provides a novel process for I thick sheets having a strong continuous resin- I impregnated surface which prevents the passage 'of moisture into the interior of the sheet. An object of this invention is to prepare paper products having improved resistance to dimensional changes produced by variations in atmos- I In the conventional I f process for the impregnation of paper with ureaformal'dehydewater-soluble resin, the resin is,

the sheet. For example, this has ben accom plished by employing a dilute solution of ureaan e and to revent rematm-e e iniflca formaldehyde resin in the beater, where the fibers d r g p p r s arehydrated, after which the fibers are fed onto preparing relatively .--j -having a. surface impregnated. .with ;urea-. g formaldehyde resin. This application is a con- 'tin'uat'ion-in-part of my copending application- S.'N. 555,042, filed September 20, 1944, wherein I' disclose that flbrous'mat'erials, particularly wood;

2o.=.of about 200'to 1000 pounds per square inch.

Application November 18, 1944, I Serial No. 564,176

1 Claim. (Cl. 117-65) '2 pheric humidity. Another obiect is to prepare paper products having 'a surface or outerv shell which is relatively impervious to moisture.

These and other objects are accomplished in ac-.

'cordance with the invention by treating paper cards or sheeting with a solution of methylol urea at a slightly alkaline pH, and thereafter subjecting the surface of the treated article under conditions hereinafter described in detail to the action of heat and pressure.

.In general to obtain the products of this invention relatively thick paper cards or sheeting '(preferably having a thickness of at least about 0.005 inch) are treated with methylol urea solutions, preferably of about 10% to 50% concentration,

for a period of time which may be as short as about 5 seconds or less, and the resulting product is heated at a temperature of about 240 to 320 F., preferably about 280 to 320.F. under a pressure The methyloi urea solutions which may be employed in the practice of thisinvention may be prepared by any suitable method, such as the "process described in the Dittmar: and, Strain patent U. S. 2,321,544. The preferred solvent is water although organic solvents, such as methanol,-ethanol, propanol, -glycols, glycol ethers, dioxolane, dioxane, and the like, they be employed if desired. The pH, should be within the neutral to slightly alkaline range, i. e. about 7.0 to 9.0, and preferably should not be substantially greater than aq. lituffers,v such as dlethanolamine, sodium? polymethacrylate, hexamethylene tetra- 1 mineftriethanolamine, monoethanolamine, sodi- .methylo'l ureas themselves.

'v, um acetatef'borax, and mono-sodium phosphate may be. employed to keep the pH within the detion ofthe methylol urea. The 'methylol urea solutions may be prepared from urea and formaldehyde, or from the The desired molar ratio oflcombined formaldehyde to combined urea in the methylol, urea solution may be obtained by adding appropriate quantities of monomethylol urea to formaldehyde or dimethylol urea to urea. For certain purposes, when maximum stability on storage is desired, the. mixture of methylolureas in solution may contain from about ,25 mol percent monomethylol urea and molper centdimethylol urea to about 75 mol percent monomethyloi urea and 25 mol per cent dimethylol urea, i. e.'the ratio of formaldehyde to meals about 1.25 to about 1.75. Foroptimum hardness and dimensional stability the ratio of formalde- The treatmentof preformed paper or cardboard 'with the methylol urea solution, in accordance with the invention, is accomplished either by coating a surface with the solution, or by dipping the paper or cardboard into the solution. .7 In this manner the weight of resin taken up is generally about"% to 15% based on the weight ofthe paper. The application of heat and pres- I sure to themethylol urea treated paper is accompushed by heating the material in a press, such as a press containing heated platens. Alternatively, the heat and pressure may be applied by passing themethylol urea treated sheet between heated rolls. Only a short curing period is required, usually not more than about 5 minutes.

The invention is further illustrated by means of the following example.

' Emample.Manila cards, 0.0067 inch thick, measuring about 8.30 centimeters across the machine direction, were dipped for 5 seconds in a 25% solution of methylol urea, having a urea: formaldehyde ratio of 1 5.1, at room temperature.

The concentration'of methylol urea solution was 25% by weight. Absorption of methylol urea solids was 11% by weight. The cards were dried and then heated at a pressure of 500 pounds per square inch at 300 F. for 5 minutes. The resultng product contained an infusible, insoluble, urea-formaldehyde resin-impregnated surface, but the resin did not penetrate the interior pore tionsof the card. After the cur n the dimension of the cards, measured across the machine direction was 8.26 centimeters. The dimension change of the card with changes in relative humidity was 30% to 50% less than the corresponding dimension change of untreated cards, within the relative humidity range of 20% to 75%.

-The 'cards'used in this experiment were not appreciably penetrated by the methylol urea solution during the 5 second dip. This fact was further illustrated in a separate test in which two of the cards, 0.0067 inch thick, were pressed face to face and immersed in the methylol urea solution for 1 hour, after which it was found that the faces in contact had not been wetted.

While in the example given above the invention is illustrated as applied to the manufacture of dimensionally stable cards, it is to be understood that many difl'erent embodiments of the invention will occur to those skilled in the art, For

- exampla'the invention may be used in the manu facture of variouspaper products which require a dimensionally stable surface, such as maps.

paperemployed in offset printing, graph paper,

cards for business machines (perforated card type), and measuring devices of various kinds requiring precise control over the surface dimensions, and resistance to changes produced by variations in atmospheric'humidity.

'Ih'e invention is therefore not limited except as set forth in the following claim.

I claim:

A process. for improving the dimensional stability of cardboard paper products adapted for use in making business machine cards of the perforated card type, which comprises dipping such cardboard having a thickness of at least 0.005 inch for not more than 5 seconds in a solution consisting essentially of water and methylolureas having a formaldehydezurea moi ratio between about 1.3and 1.6 at a pH of about 7.0.to 9.0, saidsolution having a concentration of about 10% to 50% by, weight, withdrawing the said cardboard from the said solution when the cardboard has been-only partially penetrated by .thesolution, drying the 7 resultant impregnated'cardboard andftliereafter heating the resultant dried individual sheetsof impregnated cardboard at a temperature 'of about 280 to 320 F. under .a pressure ofabout 200" to .lOOOpoun'ds. per square inch. whereby a cardboard product having a surface impregnated with infusible insoluble urea formaldehyde resin is obtained I REFERENQES acme The following references"-ar file of this patent:

record in'the Number Name Date 2,029,525 Ellis Feb. 4., 1936 2,325,302 Britt July 27, 1943 2,315,128 Newkirk Mar. so, less 2,161,805 Dreyfus-.-.. June 13,,l989 y FoaEicN- PATENTS I Number" -j fC'o untry [Date 118,436 Australia ;.-April"1944 [cr ses Great Britain July '3, 1930 

